Projection type image display device

ABSTRACT

A projection type image display device includes a plurality of image display elements that perform modulation of light rays in accordance with an image signal, a color synthesizing optical element for synthesizing light rays having mutually different wavelength ranges that have been modulated by these image display elements by use of a dichroic membrane, lens groups having a positive refractive power as a whole that are disposed between the image display element and the color synthesizing optical element, and a projection optical system for projecting an image synthesized by the color synthesizing optical element. The projection type image display device further includes a storage circuit for storing data used to correct the brightness irregularity of a projected image, and a brightness irregularity correcting circuit for correcting brightness irregularity of a projected image on the basis of the data stored in the storage circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection type image display device,such as a liquid crystal projector, including a color synthesizingoptical system that uses a dichroic membrane.

2. Description of the Related Art

Conventionally, various devices have been proposed as a projection typeimage display device, such as a liquid crystal projector, including acolor synthesizing optical system.

A conventional projection type image display device will now bedescribed with reference to FIG. 8. In FIG. 8, white light emitted froma light source 201 is transformed into substantially parallel rays oflight by a parabolic mirror 2, a reflecting mirror (not shown), and alens group (not shown).

The substantially parallel rays of light are subjected to colorseparation by dichroic mirrors 203 and 204, highly reflecting mirrors205, 206 and 207, and a lens group (not shown), and are condensed toliquid crystal panels 208, 209 and 210 disposed for each color. The eachcolor light that has entered the liquid crystal panels 208, 209 and 210is then modulated by the liquid crystal panels 208, 209 and 210displaying an image, thereby generating spatially modulated lightaccording to the display image.

Each color modulated light is subjected to color synthesis by a colorsynthesizing dichroic prism 211, and is enlarged and projected onto ascreen 213 through a projection lens group 212. Meanwhile, a group ofpositive refractive lenses 214, 215 and 216 are disposed between eachliquid crystal panel 208, 209 and 210 and the dichroic prism 211 inorder to reduce the optical system in size. Therefore, each colormodulated light in the form of substantially parallel light that haspassed through each liquid crystal panel 208, 209 and 210 is effected bythe condensing (converging) action of the positive refractive lenses214, 215 and 216, and is made non-parallel. This non-parallel lightstrikes the color synthesizing dichroic prism 211.

Therefore, disadvantageously, for the example, the incidence angle ofthe light upon the dichroic membrane has a difference between the rightand the left of the image, and thereby a difference arises in thespectral/transmitted cutoff wavelength in the dichroic membrane. As aresult, in each color light, nonuniformity (color unevenness orbrightness irregularity) in the brightness level occurs in the right andthe left of the screen.

As a solution to the color unevenness, a countermeasure has been made toform a so-called gradient membrane as the dichroic membrane in whichthickness or a refractive index slantingly varies in a specificdirection.

However, there is a problem in that, usually, color unevenness can besimply corrected only in one direction in the thus formed gradientmembrane, so that color unevenness in each direction cannot besufficiently corrected, and, in addition, a color synthesizing prismbecomes expensive due to the formation of the gradient membrane.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide aprojection type image display device capable of correcting colorunevenness that easily occurs when a lens group with positive refractivepower is disposed between a color synthesizing optical element, such asa color synthesizing prism, and an image display element by a simple,inexpensive structure, irrespective of whether a horizontal or verticaldirection in a projected image.

In order to achieve the object, according to the present invention, in aprojection type image display device that includes a plurality of imagedisplay elements for performing modulation of light rays in accordancewith an image signal, a color synthesizing optical element forsynthesizing light rays different from each other in the wavelengthrange that have been modulated by these image display elements by use ofa dichroic membrane, and a lens group that has positive refractive poweras a whole and that is disposed between the image display elements andthe color synthesizing optical element, and a projection optical systemfor projecting an image synthesized by the color synthesizing opticalelement, the projection type image display device further includes astorage circuit for storing data to correct brightness irregularity (thenonuniformity of a brightness level) of a projected image and abrightness irregularity correcting circuit for correcting brightnessirregularity of a projected image on the basis of the data stored in thestorage circuit.

Thereby, it is possible to correct the nonuniformity (color unevenness)in the brightness level that easily occurs when the lens group withpositive refractive power is disposed between the color synthesizingoptical element and the image display element by a simple, inexpensivestructure, irrespective of whether a horizontal or vertical direction inthe projected image, and is possible to project and display an imagewith high picture quality and with no color unevenness.

Additionally, when a gradient membrane is used as a dichroic membrane inorder to correct color unevenness in a specific direction, colorunevenness in a direction that cannot be optically corrected by thisgradient membrane can be electrically corrected with ease.

More specifically, the brightness irregularity correcting circuit merelyneeds to correct an input signal to the image display element in eachpixel of the image display elements or in each pixel area (i.e., pixelgroup) thereof. Other objects and structures of the present inventionwill become apparent from disclosure of the embodiments provided later.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a liquid crystal projector, which is a firstembodiment of the present invention.

FIG. 2 shows a structure of a signal-processing circuit in the liquidcrystal projector.

FIG. 3(a) and FIG. 3(b) explain the operation of a color unevennesscorrection circuit in the signal-processing circuit.

FIG. 4 shows a structure of a signal-processing circuit in a liquidcrystal projector, which is a second embodiment of the presentinvention.

FIG. 5 shows a structure of a signal-processing circuit in a liquidcrystal projector, which is a third embodiment of the present invention.

FIG. 6 explains the operation of a color unevenness correction circuitin the third embodiment.

FIG. 7 explains an example of correction data creation in the firstembodiment.

FIG. 8 shows a structure of a conventional liquid crystal projector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a structure of a liquid crystal projector (projection typeimage display device), which is an embodiment of the present invention.

First, an optical system in a liquid crystal projector 100 will bedescribed. In FIG. 1, white light emitted from a light source 1 istransformed into substantially parallel rays of light by a parabolicmirror 2, a reflecting mirror (not shown), and a lens group (not shown).

The substantially parallel light is subjected to color separation bydichroic mirrors 3 and 4, highly reflecting mirrors 5, 6 and 7, and alens group (not shown) so as to turn into each color of red, green andblue, and is condensed onto liquid crystal panels (image displayelements) 8, 9 and 10 disposed for each color.

Light rays that have entered the liquid crystal panels 8, 9 and 10 arethen modulated by the liquid crystal panels 8, 9 and 10, respectively,that display images, thereby producing spatially modulated light inaccordance with the displayed images.

The modulated light of each color is subjected to color synthesis by acolor synthesizing dichroic prism (color synthesizing optical element)11, and is enlarged and projected onto a screen 13 through a projectionlens group 12.

Two dichroic membranes 11 a and 11 b that are each inclined with respectto an optical axis are disposed in the color synthesizing dichroic prism11 without intersecting each other in the prism.

The dichroic membrane 11 a synthesizes two kinds of color-modulatedlight among red modulated light, green modulated light, and bluemodulated light that have mutually different wavelength ranges by itsreflecting/transmitting action. The dichroic membrane 11 b synthesizesthe two kinds of color-modulated light that have been synthesized by thedichroic membrane 11 a and the remaining color-modulated light by itsreflecting/transmitting action. The modulated light that has thusundergone the color synthesis is emitted from an emitting surface of theprism 11 to the projection lens group 12.

In this embodiment, positive refractive lens groups 14, 15 and 16 aredisposed between the liquid crystal panels 8, 9 and 10 and the colorsynthesizing dichroic prism 11, and thereby the optical system isreduced in size.

Herein, each color-modulated light, which is substantially parallellight and which has been emitted from the liquid crystal panels 8, 9 and10, is transformed into non-parallel light by the condensing action(converging action) of the positive refractive lens groups 14, 15 and16. This non-parallel light enters the dichroic films 11 a and 11 b ofthe color synthesizing dichroic prism 11. Therefore, for the reasongiven in the description of the conventional technique, there is a fearthat the nonuniformity of a brightness level (color unevenness) willoccur in an image projected onto the screen 13. However, according tothis embodiment, this color unevenness can be corrected regardless ofdirections on the image, as described later.

Next, a description will be given of electric-signal processing in theliquid crystal projector according to this embodiment.

An image signal, such as a video signal or VGA signal, that has beeninput to a decoder 17 from the outside is digitized by the decoder 17,and is sent to a signal-processing circuit 18.

The signal-processing circuit 18 performs digital signal processing,such as resolution transformation or frame rate transformation, for theimage signal, and performs LCD signal processing, such as gammacorrection by which the VT characteristics of the liquid crystal panels8, 9 and 10 are corrected.

The signal-processing circuit 18 further performs corrections necessaryto correct the color unevenness in a color unevenness correctionprocessing circuit (brightness irregularity correcting circuit) 19 forthe image signal to be input to the liquid crystal panels 8, 9 and 10that has undergone the aforementioned processings.

Since the image signal is corrected to cancel the color unevennessbrightness irregularity or nonuniformity of a brightness level) by thesecorrections, no color unevenness occurs in the image (projection image)formed by light rays that has been modulated by the liquid crystalpanels 8, 9 and 10 and that has been synthesized by thecolor-synthesizing prism 11.

FIG. 2 shows in detail the signal-processing circuit 18 that includesthe color unevenness correction circuit 19. Image signals (DRi, DGi,DBi) digitized by the decoder 17 are input to the color unevennesscorrection circuit 19 through the digital signal processing circuit 22for a scan converter or resolution transformation and through the LCDsignal processing circuit 23.

A frame memory 24 in which correction data used to correct the colorunevenness in horizontal and vertical directions is stored is connectedto the color unevenness correction circuit 19. The frame memory 24 ismade up of a ROM and other elements, in which the correction data isprestored.

The color unevenness correction circuit 19 reads out correction data,which synchronizes with the image signals ARi, AGi and ABi input fromthe LCD signal processing circuit 23, from the frame memory 24 by use ofa timing signal from a timing generator 21, and corrects the imagesignals ARi, AGi and ABi in real time.

FIG. 7 shows an example of a method for creation of correction data.First, with respect to each color, image signals, each having a uniformbrightness level, are sequentially input to the liquid crystal panels 8,9 and 10, and are displayed as images.

When projected onto the screen in this state, color unevenness occurs inthe projected image on the screen in a horizontal or vertical direction.Herein, brightness data in the projected image on the screen of eachcolor will be designated as Lr (x,y), Lg (x,y) and Lb (x,y), and amaximum brightness value in the brightness data of each color will bedesignated as Lr_max, Lg_max and Lb_max.

The correction data mentioned here is a gain factor for performing again adjustment for each pixel of the liquid crystal panels 8, 9 and 10in a direction where unevenness in the brightness data is canceled, andis expressed as follows:Cr (x,y)=Lr_max/Lr (x,y)  (1)Cg (x,y)=Lg_max/Lg (x,y)  (2)Cb (x,y)=Lb_max/Lb (x,y)  (3)wherein (x,y) indicates coordinates at the spatial position of eachpixel.

However, if there is no need to perform the procedure up to the gainadjustment for each pixel, the amount of correction data may bedecreased so as to reduce the size of the frame memory 24 by setting thecorrection data shown by the equations (1), (2) and (3) for eacharbitrary pixel area for simplification.

FIG. 3(a) shows an example of a correcting operation when a uniformwhite (gray) color is displayed. Since image signals input to the liquidcrystal panel is a uniform white (gray) color, the image signals ARi,AGi and ABi prior to corrections have no positional level difference,and have a constant level in both horizontal and vertical directions.

However, in order to prevent color unevenness from occurring in theprojected image as described above, the correction data is created tocancel this color unevenness according to the method shown in FIG. 7.

In other words, although the image signal has a uniform brightness level(e.g., 255), such correction data so as to enlarge the gain is set as aprojected image (modulated light) on the screen 13 in a pixel or in apixel area in which brightness becomes lower than the maximum brightnessLr_max, Lg_max and Lb_max because of the color unevenness.

FIG. 3(b) shows an example of correction data stored in the frame memory24. Image signals (after-correction signals) obtained after completionof corrections are expressed as follows:ARo (x,y)=Cr (x,y)×ARi (x,y)  (4)ABo (x,y)=Cb (x,y)×ABi (x,y)  (5)AGo (x,y)=Cg (x,y)×AGi (x,y)  (6)wherein Cr (x,y), Cb (x,y) and Cg (x,y) are red correction data, bluecorrection data and green correction data, respectively.

The after-correction signal (ARo, AGo, ABo) of each color that haspassed through the color unevenness correction circuit 19 is changedinto a signal having an inclination or variation as shown in FIG. 3(a).

Since the liquid crystal panels 8, 9 and 10 driven by use of theafter-correction signal (ARo, AGo, ABo) have space transmittancecharacteristics by which color unevenness is canceled, an image havinguniform brightness is displayed on the screen 13.

As described above, according to this embodiment, electric signals inputto the liquid crystal panels 8, 9 and 10 are corrected to eliminate thecolor unevenness of a projected image caused by an optical system, and,accordingly, the aforementioned color unevenness can be corrected by asimple, inexpensive structure, regardless of whether a horizontal orvertical direction in a projection screen.

In the above embodiment (FIG. 2), a description was given of a case inwhich color-unevenness-correction processing is applied to the outputsignal from the LCD signal processing circuit 23 by means of the colorunevenness correction circuit 19. Instead of this, as shown in FIG. 4,color unevenness correction processing can be applied to the outputsignal from the digital signal processing circuit 30 prior to the stepof being input to the LCD signal processing circuit 23.

In this case, amplifiers 25, 26 and 27 of FIG. 2 are realized bymultipliers 32, 33 and 34.

Further, in this case, since color unevenness correction processing isperformed at the stage preceding the LCD signal processing circuit 23 bywhich the VT characteristics of the liquid crystal panels 8, 9 and 10are corrected, correction data is expressed as follows:Cr (x,y)=Tvt _(—) r [Lr_max/Lr (x,y)]  (7)Cg (x,y)=Tvt _(—) g [Lg_max/Lg (x,y)]  (8)Cb (x,y)=Tvt _(—) b [Lb_max/Lb (x,y)]  (9)wherein Tvt_r [L], Tvt_g [L] and Tvt_b [L] are voltage-transmittancecharacteristic transformation functions of the liquid crystal panels 8,9 and 10.

Color unevenness correction processing may be performed at any positionbetween the input of the digital signal processing unit 22 and input ofthe LCD driving unit 20 according to methods other than the methods ofFIG. 2 and FIG. 4 as long as a correction circuit capable of gaincontrol is disposed at each spatial position of a pixel or a pixel area.

FIG. 5 shows the structure of a liquid crystal projector (projectiontype image display device), which is another embodiment of the presentinvention. In this embodiment, the same reference symbols as in theabove embodiment are given to the same constituent elements as in theabove embodiment.

In this embodiment, dichroic membranes 50A and 50B that are provided inthe color synthesizing dichroic prism 50 are each formed with a gradientmembrane in which thickness or a refractive index slantingly varies in aspecific direction. The optical system of FIG. 5 has the same structureas that of FIG. 2, except for the color synthesizing dichroic prism 50.

Since the gradient membrane are used as dichroic membranes 50A and 50B,light that passes through any position of the liquid crystal panels 8, 9and 10 becomes equal in the incidence angle upon the dichroic membranes50A and 50B in the horizontal direction that corresponds to thecharacteristic inclination direction of the gradient membrane, andtherefore color unevenness can be prevented from occurring in thehorizontal direction.

However, as in the above embodiment, an electric color unevennesscorrection is needed in the vertical direction.

When gradient membranes used for the dichroic membranes 50A and 50B arereduced in cost, there is a case in which unevenness in characteristicsoccurs, and color unevenness cannot be completely corrected even in thehorizontal direction. In this case, as in the above embodiment, colorunevenness correction processing must be electrically performed in thehorizontal and vertical directions.

FIG. 6 shows an example of a correcting operation when a uniform white(gray) color is displayed on the liquid crystal panels 8, 9 and 10.Color unevenness in the horizontal direction can be almost completelycorrected by use of a properly formed gradient membrane. However, sincecolor unevenness in the vertical direction cannot be corrected,correction data to be stored in a frame memory or a line memory 51 iscreated to cancel the color unevenness in the vertical direction.

Only one-dimensional data in the vertical direction can be roughly usedas correction data. However, if precisely corrected, two-dimensionaldata may be used as in the above embodiment.

FIG. 6 shows a case in which color unevenness correction processing isperformed only in the vertical direction. In this case, the memory 51can be formed with a line memory for a vertical line.

Image signals (after-correction signals) obtained after completion ofcorrections are expressed as follows:ARo (y)=Cr (y)×ARi (y)  (10)ABo (y)=Cb (y)×ABi (y)  (11)AGo (y)=Cg (y)×AGi (y)  (12)wherein Cr (y), Cb (y) and Cg (y) are red correction data, bluecorrection data and green correction data, respectively, stored in theline memory 50.

Since the liquid crystal panels 8, 9 and 10 driven by use ofafter-correction signals (ARo, AGo, ABo) have space transmittancecharacteristics by which color unevenness in the vertical direction iscanceled, an image having uniform brightness is displayed on the screen13.

As described above, according to this embodiment, the color unevennessof the projected image caused by the optical system is corrected byusing the gradient membrane in the horizontal direction and bycorrecting the electric signals input to the liquid crystal panels 8, 9and 10 in the vertical direction. Therefore, the aforementioned colorunevenness can be corrected by a simple, inexpensive structure.

Color unevenness correction processing may be performed at any positionbetween input of digital signal processing unit 22 and input of LCDdriving unit 20 according to methods other than the method of FIG. 5 aslong as a correction circuit capable of gain control is disposed at eachspatial position of a pixel or a pixel area.

In each of the aforementioned embodiments, a description was given of acase in which the prism 11 having the dichroic membrane is used as thecolor synthesizing optical element. However, without being limited tothis prism, the present invention can be applied to a case in whichother color synthesizing optical elements, such as an element in which adichroic membrane is provided on a plate glass, are used.

Further, in each of the aforementioned embodiments, a description wasgiven of a case in which the liquid crystal panel (LCD) is used as theimage display element. However, the present invention can also beapplied to a case in which image display elements other than the liquidcrystal panel are used.

As described above, according to all of aforementioned embodiments, itis possible to correct the nonuniformity (color unevenness) of abrightness level that easily occurs when a lens group with positiverefractive power is disposed between a color synthesizing opticalelement and an image display element by a simple, inexpensive structureirrespective of whether a horizontal or vertical direction in theprojected image, and is possible to project and display an image withhigh picture quality and with no color unevenness.

Additionally, when a gradient membrane is used as a dichroic membrane inorder to correct color unevenness in a specific direction, colorunevenness in a direction that cannot be optically corrected by thisgradient membrane can be electrically corrected with ease.

1-5. (canceled)
 6. A projection type image display device comprising: aplurality of image display elements, each of which modulates light raysin accordance with an image signal; a color synthesizing optical elementcomprising a dichroic film including a gradient film in which itsthickness varies in a predetermined direction, the color synthesizingoptical element synthesizing the light rays modulated by the pluralityof image display elements; a projection optical system projecting thelight rays synthesized by the color synthesizing optical element onto aprojection surface; and a signal processing circuit which corrects theimage signal by one of each pixel and each pixel area such thatbrightness irregularity in the predetermined direction of an imageprojected by the projection optical system is reduced or cancelled,wherein the signal processing circuit corrects the image signal suchthat the brightness irregularity, which cannot be corrected by thegradient film, is reduced or cancelled.
 7. The projection type imagedisplay device according to claim 6, wherein the signal processingcircuit corrects the image signal such that the brightness irregularityin the predetermined direction which cannot be corrected by the gradientfilm is cancelled.
 8. The projection type image display device accordingto claim 6, further comprising a positive refractive lens disposedbetween the image display elements and the color synthesizing opticalelement, wherein an incident angle onto the dichroic film of the lightrays condensed by the positive refractive lens varies in thepredetermined direction.
 9. The projection type image display deviceaccording to claim 6, wherein the dichroic film reflects a first colorlight and transmits a second light, the first color light being one ofred color light, a green color light and a blue color light withdifferent wavelength regions, and the second color light being one ofthe other two lights that are not reflected.
 10. A projection type imagedisplay device comprising: a plurality of image display elements, eachof which modulates light rays in accordance with an image signal; acolor synthesizing optical element comprising a dichroic film includinga gradient film in which its refractive index varies in a predetermineddirection, the color synthesizing optical element synthesizing the lightrays modulated by the plurality of image display elements; a projectionoptical system projecting the light rays synthesized by the colorsynthesizing optical element onto a projection surface; and a signalprocessing circuit which corrects the image signal by one of each pixeland each pixel area such that brightness irregularity in thepredetermined direction of an image projected by the projection opticalsystem is reduced or cancelled, wherein the signal processing circuitcorrects the image signal such that the brightness irregularity, whichcannot be corrected by the gradient film, is reduced or cancelled.